[CMake] Reverse logic

[Rob McDonald]

I feel like this is a really dumb question, but I've got myself wrapped
around the axel....

I'd like to use a negative conditional on a variable that may or may not be
defined...  say USE_SYSTEM_FOO

So, I'd like to do something like this...

IF( NOT ${USE_SYSTEM_FOO} )
    # Build my own FOO
ENDIF()

However, this does not work as desired.  On the other hand, the following
version does have the desired behavior (shouldn't these be the same?).

IF( ${USE_SYSTEM_FOO} )
ELSE()
    # Build my own FOO
ENDIF()

I tried a couple of combinations with DEFINED thrown in.  They didn't work
as desired either.  I didn't chase after a compound conditional like the
following simply due to ugliness....

IF( NOT DEFINED ${USE_SYSTEM_FOO} OR NOT ${USE_SYSTEM_FOO} )
    # Build my own FOO
ENDIF()

What is the CMake idiomatic way to do this?

BTW, I'm going to do a bunch of these, and I'd like to surround them with a
global USE_SYSTEM_LIBRARIES, so solutions that don't get exponentially
uglier with linear increases in complexity are desired.

Rob
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